RNA-RNA and RNA-protein interactions lie at the heart of essential steps of the eukaryotic gene expression pathway. Defects in these processes due to inherited mutations can result in degeneration of the retina, cerebellum, and other tissues. The proposed studies will result in a better understanding of RNA-based mechanisms of gene expression in both the normal and disease states. In particular, we will determine the molecular mechanism for U4/U6.U5 tri-snRNP assembly and activation in pre-mRNA splicing, and for Sen1- dependent termination of transcription by RNA polymerase II. Inherited defects in the first process can lead to autosomal dominant retinitis pigmentosa (adRP), which results in progressive blindness, and in the second process can lead to ataxia ocular apraxia type 2 (AOA2), which results in progressive muscle weakness (cerebellar ataxia). In neither case is the relationship between the molecular defect and the disease understood. Better knowledge of the molecular mechanisms of pre-mRNA splicing and transcription termination should lead to more accurate diagnosis and prognosis of these and other diseases, and may ultimately result in new therapeutic approaches. Furthermore, the proposed studies will illuminate basic mechanisms of eukaryotic gene expression that can be exploited for synthetic biology and biotechnology.